Dietary flavonoids as antioxidants

Flavonoids are ubiquitously present in fruits and vegetables. They have attracted much attention in relation to prevention of degenerative diseases such as atherosclerosis. Their antioxidant activity should be at least partly responsible for such prevention. The mechanism of antioxidant activity of flavonoids can be characterized by direct scavenging or quenching of oxygen free radicals or excited oxygen species as well as inhibition of oxidative enzymes that generate these reactive oxygen species. The essential part of the free radical-scavenging activity of flavonoids is attributed to the o-dihydroxyl group in the B ring (catechol group) in their diphenylpropane structure. Catechol typeflavonoids therefore possess powerful antioxidant activity. Conjugation of glucuronide/sulfate during intestinal absorption attenuates their antioxidant activity, but some metabolites containing an o-dihydroxyl structure, such as quercetin 3-O-beta-d-glucuronide (Q3GA), retain considerable antioxidant activity. Q3GA was found to be effective in the inhibition of lipid hydroperoxide-induced oxidative stress in the nerve cell model PC-12. Our in vivo study using high cholesterol-fed rabbits also showed accumulation of quercetin metabolites in aortic tissue, and inhibition of deposition of cholesteryl ester hydroperoxide. It is evident that quercetin metabolites are distributed in human atherosclerotic lesions, particularly the macrophage-derived foam cell. The specific target should therefore be taken into account when evaluating the antioxidant activity of dietary flavonoids in vivo.